fix Setting#find_or_default is not private

git-svn-id: http://svn.redmine.org/redmine/trunk@18694 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/setting.rb

@@ -335,4 +335,5 @@ private
     end
     setting
   end
+  private_class_method :find_or_default
 end